Japanese Patent Application Publication No. 2011-226843 discloses a sensor for detecting whether there is oxidation degradation of fuel in a fuel tank. Japanese Patent Application Publication No. 2011-226843 is hereinafter referred to as “Patent Document 1”. The sensor of Patent Document 1 includes a mesh protecting member, an ion-exchange material disposed in the protecting member, and an electrode pair disposed in the protecting member. The ion-exchange material has low electrical conductivity and the electrode pair is in contact with the ion-exchange material. In the absence of water in the fuel tank, the sensor of Patent Document 1 separates the ion-exchange material and the fuel from each other by using the protective member. Oxidization of the fuel by water in the fuel tank causes a water-soluble oxide to move from outside of the protective member to inside of the protective member. The movement of the oxide to the inside of the protective member leads to formation of a battery by the oxide and the electrode pair. The sensor of Patent Document 1 detects degradation of the fuel by connecting a voltmeter to the electrode pair and detecting the formation of the battery inside of the protecting member.